Unit Thermochemistry Heating Curves Ws 5 Answer Key
Imke
Temperature is a measure of the average kinetic energy of particles, while heat is the transfer of thermal energy between two systems or objects with different temperatures. Thermal energy always flows from areas of high temperature to areas of low temperature until they achieve thermal equilibrium.
Heat is measured in calories, and the amount of heat involved in a process is calculated through calorimetry. Specific heat, is the heat needed to raise the temperature of one gram of substance by one degree Celsius, and heat capacity, the amount of heat needed to raise the temperature of an entire object by one degree Celsius. The equation q = mcΔT is used to calculate the amount of heat absorbed or released by a system, where q represents heat, m is mass, c is specific heat, and ΔT is the change in temperature. Exothermic processes release heat and have negative q values, while endothermic processes absorb heat and have positive q values. Heating curves show how a substance's temperature changes as heat is added, displaying phase changes such as enthalpy of fusion and enthalpy of vaporization. During phase changes, the temperature remains constant, and the equation q = mL is used, where L represents latent heat, which is the amount of heat required for a phase transition to occur without changing temperature.
Heat refers to the transfer of thermal energy between two substances due to a temperature difference, while temperature is a measure of the average kinetic energy of the particles in a substance. In a heating curve, heat is the energy applied to a substance causing its temperature to change and undergo phase transitions.
Specific heat is the amount of heat required to raise the temperature of one unit mass of a substance by one degree Celsius, while heat capacity refers to the heat required to raise the temperature of any given amount of a substance by one degree Celsius. These properties determine the slope of the heating curve during the phases when temperature changes: a substance with a high specific heat or heat capacity will require more heat to change its temperature, resulting in a shallower slope on the curve.
Calorimetry is the experimental technique used to measure the heat transfer between substances during chemical reactions or phase transitions. By performing calorimetry experiments, one can obtain data regarding the heat exchanges involved in a heating curve, such as the amount of heat needed for a phase change or temperature increase. This data can then be used to analyze, understand, and predict heating curve behavior for different substances and conditions.
Exothermic processes release heat, while endothermic processes absorb heat. These processes, along with the concept of enthalpy, help in understanding how heat is transferred or released during phase transitions in a heating curve. An increase in enthalpy corresponds to an endothermic process, where heat is absorbed by the substance, whereas a decrease in enthalpy indicates an exothermic process, where a substance releases heat. In a heating curve, enthalpy changes occur during phase transitions, as heat is either absorbed or released while the substance undergoes the state change.
In this section, we continue analyzing phase diagrams (plots of pressure vs. temperature) and correlate them to the heating curves (plots of temperature vs. energy) that you learned about earlier in the semester. We will also continue to use the phase diagram to understand the conditions needed for phase transitions. Finally, we will look at the heat of vaporization and the relationship between heat of vaporization and temperature and pressure.
The previous chapter detailed phase diagrams of pressure vs. temperature. In an earlier module, you learned about heating curves. These two types of plots provide complementary information, as seen in Figure 1. A heating curve is constructed by measuring the temperature of a substance as heat is added at constant pressure. In Figure 1A the pressure at which these measurements of water are made is 1 atm. Recall that the horizontal line segments on this type of plot indicate a phase change, and so both solid and liquid or liquid and gas phases are present as the water melts or vaporizes (light blue and green horizontal lines, respectively), but the temperature of the water does not change until the phase transition is complete.
Now consider the phase diagram in Figure 1B that you saw in the previous section. The arrows trace a path of increasing temperature at a constant pressure, the same pressure as used in the heating curve. As heat is added to ice, the temperature begins to increase (red lines in both plots). Once the temperature reaches 0 C, a phase change begins as the ice melts (represented by the blue dot on Figure 1B; the size of the dot is only chosen for clarity because the phase transition occurs only at the pressure and temperature of the phase boundary). Since Figure 1B is only indicating the temperature and not the amount of heat added, the temperature remains constant as the phase change occurs, so the dot is used to indicate that the temperature is not changing during the phase change. Only once all the ice has melted will the water, now in the liquid phase, begin to increase in temperature again (orange arrow) as the pressure remains constant. Once 100 C is reached, another phase change will occur (green dot).
While anywhere along the line segment BD represents a phase change from solid to liquid, and points Y and Z are both on that line, the correct answer is D. At point Y, the phase change is occurring at the same pressure (1 atm) that was used to construct the heating curve. The arrow X represents both a temperature change and a phase change.
As seen in the previous sections, phase diagrams contain a wealth of information about the physical states of a substance. For example, if given a temperature and pressure, you could use a phase diagram to determine the phase of that substance. If you were provided a pressure, you could use a phase diagram to determine at what temperature a phase transition will occur. You can even predict if or which phase transition will occur when heat is added to the substance.
The phase diagram for CO2 is provided in Figure 3. A sample of CO2 is at 20 C and 1 atm. Will a phase change occur if you slowly increase the pressure on CO2 but keep the temperature constant? If so, which phase change?
Yes, at 20 C and 1 atm, CO2 exists as a gas in your original sample. If the temperature remains constant and we increase the pressure, we would trace a path upward on the graph. We would eventually reach the phase boundary between the gas and liquid phases and the gas would condense to a liquid.
Set up. The phase diagram for CO2 presented in Figure 3 indicates that at -78.5C and 1 atm of pressure, CO2 exists as a solid (which we call dry ice). As dry ice warms, it will sublime and form CO2(g). In this demonstration, dry ice is placed in a constant-volume container and allowed to warm. A pressure gauge is attached to the container to measure the pressure inside the container.
Explanation. Crushed dry ice begins to sublime as the container begins to warm. As it sublimes, the presence of CO2(g) causes the pressure inside the container to increase. The system is following the solid-gas coexistence curve of the phase diagram since both solid and gas are present in the container. Eventually, the pressure and temperature inside the container reach the triple point, where solid, liquid, and gas can coexist. You can see the liquid CO2 sitting on top of the solid CO2 in the video. At the end of the video, the container is vented, releasing the built up pressure and causing the solid and gas coexistence to be reestablished.
Set up. The following video demonstrates how water expands as it freezes. A cast iron container is placed in an ice water bath and filled with ice water. A lid is tightened onto the container, making this a constant-volume container. The container is then placed in liquid nitrogen to cool it (the boiling point of liquid nitrogen is -196 C [-320 F]). A box is placed over the container to contain any shrapnel from the container.
Phase diagrams (plots of pressure vs. temperature) were correlated with heating curves (plots of temperature vs. energy). These two types of plots provide complementary information on the phase transitions of substances. While a heating curve provides information on the phase changes at a single pressure, the phase diagram depicts the phase changes at all temperatures and pressures. Phase diagrams can be used to determine and predict the phase of a substance at a given temperature and pressure and also the phase changes that will occur. One of these phase changes is vaporization, which is an endothermic process that transforms a liquid to a gas. We use the Clausius-Clapeyron equation to relate the enthalpy of vaporization with the vapor pressure of a substance.
c80f0f1006